Read JSON instances from a stream

[fontanf]

Motivation

The instance readers only took a file path. To read an instance from a string, e.g. in the WebAssembly module where the instance comes from JavaScript, you had to write it to a file first.

Change

• C++: each InstanceBuilder (all six types) gets a read(std::istream&) overload that reads an instance in the JSON format from a stream. read(instance_path) opens the file and calls it, so the format and behaviour are unchanged.
• Python: InstanceBuilder.read accepts a path, a path-like object (e.g. pathlib.Path) or a text stream with a read() method (e.g. io.StringIO). Same convention as Solution.write.
  • The bindings used to take the address of InstanceBuilder::read, which is ambiguous now that it's overloaded. They go through two helpers in python/src/common.hpp instead.
• Doc fix: irregular's read(instance_path) comment said "Read item types from a file".

Testing

• New test_read_stream in python/tests/test_instance.py, for each problem type:
  • the same JSON instance read from a path, an io.StringIO and a pathlib.Path gives the same instance;
  • invalid JSON raises an error;
  • an argument that's neither a path nor a stream raises TypeError.
• C++ unit tests: 693 passed. Python tests: 362 passed, 13 skipped.